This invention relates to ultrasonic diagnostic imaging systems and, in particular, to synthetically focused ultrasonic diagnostic imaging systems which image tissue, flow, and flow spectra.
Today""s conventional ultrasound systems produce images by scanning a region of the body with ultrasound beams electronically formed by a transducer array. The timing of signals applied to the array elements determines the direction in which a beam is transmitted and the depth of focus of a beam. A plurality of adjacent beams are transmitted to adequately spatially sample the target region. The echoes from along the beam directions are processed in known ways to form an image of the plane or volume scanned by the beams and depict characterstics of the tissue or bloodflow within the plane or volume.
When a beam is transmitted, the beam profile determined by the transducer element timing can be set to produce the best focus around a selected focal point at a desired depth. A transmitted beam can have only one such focal point; once the beam is launched, the focal characteristic cannot be changed or extended. The clinician will generally set the focus to the depth at which the anatomy of interest may be found. If the clinician wants the optimal focus to be at two or more depths or range of depths, multizone focus must be used, whereby two or more beams are transmitted in the same direction, each focused at a different depth. The entire image plane or volume is scanned by multiple beams in each direction, and the best focused portions of the sets of beams are spliced together to produce a composite image which exhibits the optimal focus at a plurality of depths. However, each differently focused beam set will extend the time required to acquire the image information, which reduces the frame rate of display. Going from one focal zone to two can approximately halve the frame rate, for example.
During reception, optimal focus can be obtained dynamically over the full imaging depth. This is because the receive focus can be dynamically adjusted electronically by continually adjusting delays in the receive beamformer as echoes are received over the full range of depth. The transmitted beams are constrained to a single focal point or region since the transmit focus is accomplished acoustically and not electronically.
A technique for overcoming this transmit focus limitation is known as synthetic focus. Synthetic focus is described in U.S. Pat. No. 4,604,697, for instance. In a synthetic focus system, each transducer element or subset of transducer elements is actuated sequentially. The transmission from each element or group of elements is not focused acoustically but covers the entire image region. The echoes from each transmission are received by all of the elements concurrently and stored. These echoes are then combined in different combinations with different effective delays, thereby forming coherent echoes at points in the image region which are effectively focused at all points. This technique produces image data which is in effect optimally focused on both transmit and receive, providing images which are optimally focused throughout the entire depth of field. In such an approach it is not necessary to first beamform onto distinct A-lines, although this can be done. Rather, the echo data can be directly reconstructed onto image pixels.
While the ""697 patent illustrates a synthetic focus technique which is limited to imaging stationary tissues, it is also desirable to extend the technique to flow imaging. U.S. Pat. No. 5,349,960 illustrates one extension of a synthetic focus architecture to Doppler. In this patent a synthetic focus acquisition subsystem is used to produce real and imaginary parts of a complex signal, which is then Doppler processed. The full synthetic focus data set is not used. Rather, reception only occurs on the transmitting element. It is desirable to have a synthetic focus architecture which utilizes more than one receive element and which produces data for all ultrasonic modes, including B mode, colorflow, and spectral flow, without the need for hardware or software dedicated to only a specific mode of operation.
In accordance with the principles of the present invention a one or two dimensional transducer array is operated to acquire synthetic focus ultrasound signals by actuating one or a subset of the elements of the array and receiving echo signals on all of the array elements in response to a transmit event. The received signals are digitized and stored in a memory. Sets of delays are applied to combinations of the stored signals to produce motion maps for a plurality of different velocities. One or more of the motion maps are used to produce B mode, colorflow, spectral, and other ultrasound image modes which cannot be produced by conventional ultrasound systems. The images, either two or three dimensional, are in optimal focus throughout the image plane or volume and can be produced at high frame rates of display. Various transducer element sequencing may be employed to increase the frame rates even higher.